worktree: add -b/-B options

One of git-worktree's roles is to populate the new worktree, much like
git-checkout, and thus, for convenience, ought to support several of the
same shortcuts. Toward this goal, add -b/-B options to create a new
branch and check it out in the new worktree.

(For brevity, only -b is mentioned in the synopsis; -B is omitted.)

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -9,7 +9,7 @@ git-worktree - Manage multiple worktrees
 SYNOPSIS
 --------
 [verse]
-'git worktree add' [-f] [--detach] <path> <branch>
+'git worktree add' [-f] [--detach] [-b <new-branch>] <path> <branch>
 'git worktree prune' [-n] [-v] [--expire <expire>]
 
 DESCRIPTION
@@ -64,6 +64,14 @@ OPTIONS
 	is already checked out by another worktree. This option overrides
 	that safeguard.
 
+-b <new-branch>::
+-B <new-branch>::
+	With `add`, create a new branch named `<new-branch>` starting at
+	`<branch>`, and check out `<new-branch>` into the new worktree.
+	By default, `-b` refuses to create a new branch if it already
+	exists. `-B` overrides this safeguard, resetting `<new-branch>` to
+	`<branch>`.
+
 --detach::
 	With `add`, detach HEAD in the new worktree. See "DETACHED HEAD" in
 	linkgit:git-checkout[1].
@@ -133,8 +141,7 @@ make the emergency fix, remove it when done, and then resume your earlier
 refactoring session.
 
 ------------
-$ git branch emergency-fix master
-$ git worktree add ../temp emergency-fix
+$ git worktree add -b emergency-fix ../temp master
 $ pushd ../temp
 # ... hack hack hack ...
 $ git commit -a -m 'emergency fix for boss'

builtin/worktree.c

@@ -126,15 +126,22 @@ static int add(int ac, const char **av, const char *prefix)
 {
 	struct child_process c;
 	int force = 0, detach = 0;
+	const char *new_branch = NULL, *new_branch_force = NULL;
 	const char *path, *branch;
 	struct argv_array cmd = ARGV_ARRAY_INIT;
 	struct option options[] = {
 		OPT__FORCE(&force, N_("checkout <branch> even if already checked out in other worktree")),
+		OPT_STRING('b', NULL, &new_branch, N_("branch"),
+			   N_("create a new branch")),
+		OPT_STRING('B', NULL, &new_branch_force, N_("branch"),
+			   N_("create or reset a branch")),
 		OPT_BOOL(0, "detach", &detach, N_("detach HEAD at named commit")),
 		OPT_END()
 	};
 
 	ac = parse_options(ac, av, prefix, options, worktree_usage, 0);
+	if (new_branch && new_branch_force)
+		die(_("-b and -B are mutually exclusive"));
 	if (ac != 2)
 		usage_with_options(worktree_usage, options);
 
@@ -145,6 +152,10 @@ static int add(int ac, const char **av, const char *prefix)
 	argv_array_pushl(&cmd, "--to", path, NULL);
 	if (force)
 		argv_array_push(&cmd, "--ignore-other-worktrees");
+	if (new_branch)
+		argv_array_pushl(&cmd, "-b", new_branch, NULL);
+	if (new_branch_force)
+		argv_array_pushl(&cmd, "-B", new_branch_force, NULL);
 	if (detach)
 		argv_array_push(&cmd, "--detach");
 	argv_array_push(&cmd, branch);